U.S. Pat. No. 7,106,032 discloses a regulated power supply circuit with two parallel output transistors: a low power output transistor and a high power output transistor, the high power output transistor being switched on only when a high output current must be supplied. The output transistors are PMOS transistors with their channels coupled in parallel between the positive power supply source and the regulated output of the power supply circuit. The power supply transistors provide for a voltage drop between the positive power supply source and the regulated output. The output voltage at the regulated output is regulated by measuring this output voltage and using the measured output voltage to control the current through the output transistors.
Thus, the output transistors dissipate the power associated with excess supply voltage. In addition the circuits in the feed-back loop that drives the output transistors dissipate power. U.S. Pat. No. 7,106,032 teaches that the latter power dissipation can be reduced by using only the low power output transistor when high power output is not required. As disclosed by U.S. Pat. No. 6,469,480 this type of circuit can also help to prevent overshoots by activating only an output transistor with low driving power in the case of a large difference between the actual output voltage at the regulated output and the desired output voltage. When this difference between the actual and desired output voltage is small, both transistors are activated to provide for a fast response.
Power supply circuits may be integrated together with other circuits, such as processors, communication transmitters and receivers, sensors etc. In this case, heating due to the power dissipation by the regulated power supply circuit may detrimentally affect the operation of such co-integrated circuits.
Such detrimental effects can be avoided, or at least reduced, by using a regulated power supply circuit with output transistors outside the integrated circuit, i.e. by integrating only the control part of the regulated power supply circuit. This makes it possible to provide for some thermal isolation between the integrated circuit and the output transistors. However, this solution introduces a problem of potential instability of the regulated power supply circuit, because the designer of the integrated circuit cannot know the parameters of the output transistors that may be selected by end users and because the wiring between the integrated circuit and the output transistors may introduce additional phase shift in the control loop.